The correct option is A realise
Option B is correct. It is in the simple present tense, which is used to express thoughts or beliefs, or a dream, like that of becoming a teacher. Option A is also in simple present tense, however, the presence of "to" makes the verb an infinitive and an "s" is not added after an infinitive. Therefore, it is incorrect. Option C is in present continuous tense and "ing" is not added after an infinitive verb. Option D is in the past tense but the indicated action or belief will occur in future, hence it is incorrect.